openvpn: revise sample configuration

openvpn: revise sample configuration

Update the openvpn sample configurations to use modern options in favor
of deprecated ones, suggest more sane default settings and add some
warnings.

* Add tls_crypt and ncp_disable to the sample configuration
* Replace nsCertType with remote_cert_tls in client sample configuration
* Comment out "option compress", compression should not be preferred
* Advise 2048-bit Diffie-Hellman parameters by default
* Add warnings about compression and use of Blowfish (BF-CBC)

Signed-off-by: Magnus Kroken <mkroken@gmail.com>